A new kind of pH and temperature sensitive material is reported. It is composed of dye-doped polymer nanoparticles incorporated into a thin film of a polyurethane hydrogel. The new pH/temperature-sensitive nanoparticles are obtained by post-staining oxygen-impermeable amino-functionalized polyacrylonitrile nanoparticles with a long-lifetime reference dye. Staining is followed by covalently linking fluorescein isothiocyanate onto the surface of the nanoparticle. The new sensor material has several distinct features: a) it enables imaging of pH via time domain dual-lifetime referencing; b) effects of temperature on pH sensing may be compensated for; and c) temperature can simultaneously be visualized via rapid lifetime imaging. The new material enables referenced and temperature-compensated pH imaging with superior spatial resolution due to the use of nanosized sensor nanoparticles.